This disclosure relates to ball joint assemblies and, more particularly, to ball joint assemblies having one or more friction coated components and methods for assembling a ball joint assembly having defined gaps within the ball joint assembly.
Ball joints have been widely used in various mechanical systems, such as automotive chassis systems, aircraft landing gear systems and agricultural machinery. The fundamental function of a ball joint is to connect two mechanical parts together while allowing relative rotational or rocking movement. Friction torque exists in any ball joint generally due to mechanical interference. Because friction torque in a ball joint often times has negative influence on the dynamic behavior of the mechanical system, there is generally a desire to minimize or reduce such friction torque. However, in certain assemblies, friction torque in a ball joint can be helpful for damping relative motion between ball joint components or adding resistance to the relative motion. In such cases, stable, predictable, and controllable friction torques are desired.
A conventional ball joint normally includes a ball stud positioned within a cavity defined by a housing. A ball race is positioned within the cavity at a first end of the housing and configured to cooperate with a ball portion of the ball stud. Grease or another liquid lubricant is applied between the ball portion of the ball stud and the ball race to reduce the friction torque between the ball stud and the ball race and improve the durability of the ball joint. In certain conventional ball joints, a second end of the housing is enclosed to prevent the grease or other lubricant from undesirably leaking from the housing.
In these conventional ball joints, the friction torque behavior of the ball joint varies with changes in properties of the grease, which can be greatly affected by temperature and/or pressure or force acting on the components. In many conventional ball joints, when the materials, dimensions, and/or surface roughness of the ball stud and/or the ball race are fixed, the friction coefficient between the ball stud and the ball race may depend on one or more factors, such as the grease viscosity, the normal load between the ball stud and the ball race, and the relative velocity between the ball stud and the ball race, for example.
As a result of these factors, there are a few inevitable drawbacks in establishing and/or controlling the friction torque of many conventional ball joints. One drawback is that the friction torque of the ball joint varies greatly with temperature. Usually, when temperature decreases, the grease viscosity increases and hence the friction torque increases. A second drawback is that the grease film thickness decreases and the friction torque of the ball joint increases with time when the ball stud continuously rotates or rocks relative to the ball race under a normal load applied between the ball stud and the ball race. A third drawback is that, before a sufficient grease film is formed, a breakaway friction torque is relatively high due to the increased friction coefficient between the ball stud and the ball race before the ball stud starts to move relative to the ball race. These drawbacks can cause an unstable friction torque, an unpredictable or uncontrollable friction torque, and/or severe wear on the ball stud and/or the ball race when a high normal load is applied between the ball stud and the ball race for a substantial period of time.
Therefore a need exists for a ball joint having a surface coating which allows the ball joint to have a friction torque which is stable both under varying temperatures and with continuous relative motion between the ball stud and the ball race under a high normal load. Further, there exists a need for a predictable friction torque of the ball joint for a given normal load applied between the ball race and the ball stud that is controllable by varying the normal load applied between the ball race and the ball stud.